Traditionally, antennas for mobile phones are either a helix or a whip or some combination thereof. These antennas were measured and compared to each other using gain tests from an anechoic chamber. A gain test in an anechoic chamber consists of measuring a single polarization (sometimes two—vertical and horizontal) with a directional antenna (reference antenna). The walls of the anechoic chamber are covered with an absorbing material to prevent reflections between the reference antenna and the mobile phone. These external antennas 100 (helix, whip, or combination) arranged on a mobile phone 110 tend to perform very well inside the anechoic chamber as they receive one of the polarizations 120 very well, as shown in FIG. 1.
In FIG. 2 an internal antenna 200 is illustrated, which usually is mounted above a ground plane, i.e. the PCB 230 of mobile phone 210, and is able to receive several polarizations 220 of an incoming wave. This is due to the electric and magnetic field distributions across the top of the antenna. In limited (only two to three planes measured) anechoic chamber measurement tests, the internal antennas demonstrate low performance. A complete anechoic chamber test, where ten or more planes are measured, the internal antennas have similar performance to the external antennas.
The typical environment where the mobile phone is used is very different from the anechoic chamber. In an urban environments schematically shown in FIG. 3, the field from the base station 350 (reference antenna) gets scattered through buildings 360, vehicles, people, etc. This scattered field that arrives at the mobile phone is comprised of several polarizations and magnitudes (changing in a random fashion). The phase of the incoming signals changes, sometimes adding to give a strong field, sometimes cancelling out completely. In addition, the phones are next to a user's head with their hand in the near vicinity. The head and the hand disturb the near fields of the mobile phone, changing the input impedance, and the far fields by absorbing some of the radiation emitted from the mobile phone.
Since the anechoic chamber does not provide sufficient information about the phone's performance in the actual environment, mobile phone manufacturers measure their phones in the field (called a field test). This involves programming the phone to send received power levels (or other performance measurements such as BER to a computer. The phone is then tested in several areas and several thousand samples are taken. This measurement typically takes several hours. Furthermore, it is not repeatable at a later time since some of the scattering objects are moving (i.e. Vehicles and people).
Several measurement systems on the market are intended for measurement of EMC and are not suitable for mobile phone measurements. These EMC scattered field boxes have no absorbing or lossy material within the chamber. If a mobile phone was placed inside this EMC chamber, the high densities of electrical field strength would disturb the near-fields of the mobile phone, changing the input impedance. Other scattered field systems have been proposed, but they rely on a single fan (or pendulum or other method of “stirring the modes”) with low losses (ie. highly conductive-like metal).